1. Field
Example embodiments are generally directed to communication networks, and more specifically, wireless communication networks involving serving and collaborating base stations.
2. Related Art
Using coordinated multipoint transmission reception (CoMP) technology, a group of base stations (BS) can potentially transmit to user equipment (UE). The group of base stations that potentially transmit to a given UE is known as the CoMP cooperating set of the given UE. During actual transmission, one or more of these BS in the cooperating set can transmit to a given UE. The other BS in the cooperating set may be silent (e.g., not transmitting) or transmitting to other UEs, which may cause interference to the given UE. The state of each BS (e.g., transmitting, not transmitting) may change over time. The UE may measure the channel and interference conditions for a given transmission, so that the UE can measure the downlink signal to interference plus noise ratio (SINR). The SINR may be used to pick a CQI (channel quality indicator) value, which is a modulation scheme with a coding rate.
The Long Term Evolution (LTE) transmit signal is two dimensional in time and frequency, and is composed of multiple time resource elements (RE). Each RE is a two dimensional tile with a duration of roughly 71 microseconds in time and a frequency of 15 kilohertz (KHz). For purposes of channel and interference measurement leading to CQI calculation, the BS can configure certain REs with pilot signals. These REs are called channel state information reference signal (CSI-RS) resources. The transmission can be explained by the following equation which is the received signal y in a given CSI-RS resource.y=Hs+I+n   (1)
where H is the channel to be estimated, s is the transmit pilot signal which is known at the transmitter and receiver. I+n is the interference and noise as measured by the UE. The UE first estimates H from received y and from s. Let the estimate be Hest. The UE then subtracts y−Hest*s to estimate the interference plus noise power (I+n)est. The estimate for the CQI value is
                    cqi        ≈                              log            2                    (                      1            +                                                                                                                        H                      est                                        *                    S                                                                    2                                                              (                                      I                    +                    N                                    )                                est                                              )                                    (        2        )            
The accuracy of the channel estimation tends to increase when the interference decreases. The theoretical ideal channel estimation occurs when only the BS whose downlink channel is to be estimated is the only BS transmitting, with all of the other BS's in the cooperating set muted (silent, not transmitting). However this channel estimation comes at the cost of inadequate interference measurement. For example, in a case where only one BS is transmitting to the UE with the remaining BS's in the cooperating set being muted, the UE can only measure the background noise power. The inadequate interference measurement can lead to inaccurate CQI computation for subsequent data transmissions when the other BS in the cooperating set are actually transmitting.